Low molecular weight polymers and copolymers of the monomers set forth herein are useful as scale inhibitors, dispersing agents for paints, sequesterants, as detergent builders and for many other uses. When a polymer, such as polyacrylic acid, is prepared, one method of preparation is the use of a Redox catalyst system (initiator and reducing agent). Because it is believed that the use of a Redox catalyst system will result in a high molecular weight polymer (e.g. in excess of an average molecular weight of 50,000), when a low molecular weight polymer is desired, a chain transfer agent, such as a mercaptan, is incorporated during the polymerization reaction to control the molecular weight and to obtain low molecular weight materials. The use of mercaptans to control molecular weight is expensive and the mercaptan odor is undesirable and permeates the resultant product.
U.S. Pat. No. 3,635,915 discloses a copolymer of maleic acid and acrylic acid prepared by polymerizing a mixture of the monomers in water in the presence of a Redox polymerization catalyst system. This patent teaches that the polymerization reaction may be terminated in order to obtain products having desired viscosities by introducing short-stopping agents when the desired extent of polymerization has been obtained. The short-stopping agents which are set forth are methyldithiocarbamic acid, hydroquinone and, when a peroxide or persulfate is utilized as a catalyst, sodium bisulfite is utilized as the short-stopping agent.
U.S. Pat. No. 3,635,915 also teaches that copolymers of maleic acid and acrylic acid may be prepared using a thermal polymerization process. In this thermal process, the catalyst, which supplies free radical, has a short half life at elevated temperatures so that, at elevated temperatures the catalyst will decompose, thereby supplying the free radicals necessary for the polymerization reaction. The thermal process of U.S. Pat. No. 3,635,915 is ineffective in preparing low molecular weight polymers without the use of a chain transfer agent. Additionally, in order to prepare polymers of exceedingly low molecular weights, excessively large amounts of the thermally decomposable catalyst and of the chain transfer agent must be used. These excessive amounts of catalyst and chain transfer agent render this thermal process economically unsound.
U.S. Pat. No. 3,509,114 is directed to the polymerization of water-soluble acrylic monomers in aqueous solution. The patent sets forth that a Redox catalyst system may be used to prepare high molecular weight acrylic polymers.
U.S. Pat. No. 4,024,099 is directed to the preparation of very high molecular weight, water-soluble acidic polymers using a Redox catalyst system and a methylolamide or formaldehyde reagent. The patent sets forth that high catalyst to monomer ratios in polymerization reactions cause lower molecular weights in the resulting polymer. The patent also teaches that the reducing agent of the Redox catalyst system may be a metal salt such as ferrous sulfate and that the reducing reagents are used in amounts approximately equivalent to that of the peroxy catalyst.
The processes described in U.S. Pat. Nos. 3,635,915; 3,509,114; and 4,024,099 are unsuitable for preparing water-soluble low molecular weight polymers either because they are more expensive processes than the present invention or they result in the preparation of high molecular weight polymers.
U.S. Pat. No. 2,789,099 is directed to a process for polymerizing acrylic acid to yield water-soluble polymeric products. Although U.S. Pat. No. 2,789,099 is silent concerning the molar ratio of initiator to metal ion, based on calculations of the examples of U.S. Pat. No. 2,789,099, the examples disclose a molar ratio of initiator to metal ion of from 6:1 to 117,000:1. U.S. Pat. No. 2,789,099 never recognized the importance of a molar ratio of initiator to metal ion of from about 10 to 1 to about 150 to 1 in controlling the molecular weight of the polymer of the aqueous polymer solution. Further, all of the examples set forth in U.S. Pat. No. 2,789,099 show the preparation of polymers having molecular weights exceeding 50,000 based on the reported viscosities and solids content. In addition, U.S. Pat. No. 2,789,099 fails to teach or suggest that the water-soluble monomer be present in the reaction solution in an amount of from about 40% to about 60%, by weight, based on the total weight of the reaction solution.
It is highly desirable to provide an aqueous polymer solution which has a high solids content (i.e., from about 40% to about 60% by weight of the entire composition) and a low average molecular weight of up to 50,000 without introducing thioether terminal groups onto the polymer. These thioether terminal groups have an objectionable odor which makes the use of the mercaptan, which supplies the terminal thioether groups, undesirable.
It is an object of this invention, therefore, to provide a process for the preparation of a water-soluble low molecular weight polymer or copolymer, as defined herein, wherein a catalyst system is used and the molecular weight of the resultant polymer is controlled by the catalyst system itself.
Another object of this invention is to provide a process for the preparation of low molecular weight, water-soluble polymers and copolymers and controlling the molecular weight of the resultant polymer by the molar ratio of initiator to metal ion and the amount of initiator present.
A further object of this invention is to provide a process for the preparation of a low molecular weight, water-soluble polymer or copolymer wherein a mercaptan or other chain transfer agent to control molecular weight is not necessary.
An additional object of this invention is to provide an aqueous polymer solution having a high solids content and a low molecular weight and devoid of a disagreeable odor due to the presence, on the polymer, of terminal thioether groups.
Other objects and advantages will become apparent from the following more complete description and claims.